Musnad Ahmad 658:98

Waki’ narrated to us, from Isra’il, from Abu Ishaq, who said: I heard Al-Bara’ say: A man from the Ansar came to the Prophet, peace and blessings of Allah be upon him, covered in iron (armor), and said: "O Messenger of Allah, shall I embrace Islam or fight?" He said: "No, rather embrace Islam, then fight." So he embraced Islam, then fought, and was killed. The Messenger of Allah, peace and blessings of Allah be upon him, said: "This one did a little and was rewarded much."